The cerebellum is one of the earliest brain regions to develop, but it is mature many months after birth. This long developmental period makes it susceptible to pre- and post-natal disruptions. It also makes it highly amenable to study; its anatomical development is well described. However, genetic control of transitions from cerebellar stem cells to mature neurons remains largely unknown. The control granule cell generation, the most numerous CNS neurons, will be examined. The cerebellum arises from two germinal zones: The rhombic lip, giving rise to granule cells, and the ventricular zone, giving rise to all other cells. Cells proliferate at the rhombic lip, migrate to the external germinal layer (EGL), proliferate as granule cell precursors (GCPs) and migrate past the Purkinje neurons as they become mature granule cells. We hypothesize that key regulatory signals underlying the shift from proliferation to differentiation can be identified by comparing the genetic changes occurring at the rhombic lip and the EGL. We know that Sonic hedgehog (Shh) is a potent mitogen for GCPs, which express the Shh receptor, Patched1 (Ptcl). Reduced ptcl function may give rise to abnormal proliferation of GCPs, causing meduloblastoma. Using Shh we have already identified gene expression changes that occur as GCPs go from being non-proliferating to proliferating. A role for Shh and Ptcl earlier, at the rhombic lip, has not yet been explored. The transition from self-renewing precursor to differentiating granule cell will be analyzed by: 1) Characterizing the phenotypic potential of cells isolated from the cerebellar primordium and rhombic lip, 2) elucidating the genetic program controlling the transition points between these stages, and 3) testing genes identified as potential regulators to determine how progenitor cells cease proliferation and start to differentiate. Ultimately, to control neuronal precursor cells in a therapeutic, directed manner in any region, from cerebellum to cortex, we must understand the mechanisms underlying the shift from proliferation to differentiation. My goal is to develop molecular biology skills to complement my previous physiology and anatomy training. As a neonatologist, I will bring these tools to bear on critical issues in neurodevelopment.